The steam generators of pressurized-water nuclear reactors comprise a tube plate of great thickness, usually exceeding 500 mm, having several thousand holes, in each of which is fastened one end of a tube of the steam generator. Each of the holes passing through the tube plate opens at one of its ends into the inlet face of the tube plate, inside the water box of the steam generator.
When the steam generator is in operation, the tubes undergo corrosion which is generally more pronounced in the region of the tubes which is adjacent to the outlet face of the tube plate. In order to conduct an examination of the regions of the tube which have undergone corrosion and to evaluate how far this corrosion has advanced, it may be necessary to extract the part of the tube located inside the tube plate and a more or less long part of the tube located beyond the outlet face, depending on the location of the regions which have undergone maximum corrosion and depending on the types of checks to be carried out on the tubes.
When one or more tubes have been extracted, it is necessary to ensure that the hole passing through the tube plate is plugged at the location of each of the extracted tubes, before the steam generator is put into operation again. For each of the extracted tubes, a sleeve is therefore installed in the passage hole of the tube, and this sleeve is fastened in the tube, for example by expansion. A plug consisting of a tubular piece having a closed bottom is then fastened inside the sleeve on the same side as the inlet face of the tube plate. The plug, which has a diameter less than the inside diameter of the sleeve after it has been installed in the tube plate, is subjected to diametral expansion which ensures that it is retained inside the sleeve. To complete the plugging of the passage hole in the tube plate, the end of the plug is welded to the end of the sleeve, these two ends virtually coinciding with one another and being located a short distance from the inlet face of the tube plate.
The plug installed in the sleeve can be a conventional mechanical plug. In this case, only the welding of the sleeve to the tube plate is carried out.
The welding of the ends of the plug and/or of the sleeve of circular shape requires the use of a rotary or orbital welding apparatus. There must be the possibility of conducting the welding operations by remote control, in view of the fact that the work is carried out inside the water box of the steam generator, i.e., in an irradiated zone where any human involvement must be avoided or limited to as short a time as possible.
There is a known automatic welding head of the T.I.G. type which makes it possible to carry out an operation to weld the end of a tubular piece located inside a sleeve or a tube. This welding head comprises a centering bush and a welding electrode mounted rotatably about the axis of the bush.
The centering bush is engaged in the tubular piece, in such a way that the axis of the bush and the axis of the tubular piece coincide. The welding electrode is placed near the end of the tubular piece, and rotating it about the axis of the bush makes it possible to carry out the welding of the end of the tubular piece.
However, to obtain a uniform and sealing weld of very high quality, the end of the electrode must move in a plane perfectly parallel to the plane of the circular end of the tubular piece and at a short and absolutely constant distance from this end of the tubular piece forming the welding bevel.
It is therefore necessary to ensure remotely and automatically that the welding machine is positioned very accurately and fastened very firmly in relation to the tubular piece to be welded.
There are known carrier devices movable inside the water box of a steam generator, so as to come into treal alignment with each of the inlet orifices of the tubes of the steam generator in order to present a tool at these orifices successively. However, such a carrier device does not make it possible to ensure rapid positioning and perfect fastening of a welding head in the region of a tubular piece to be welded inside a tube passage hole in the tube plate.
There is also a known means for fastening a tool to the inlet face of a tube plate, comprising a cylindrical part formed in the manner of a mandrel by several deformable sectors allowing diametral expansion to ensure that the cylindrical part is fastened in the tube. The deformable sectors can be put into a retracted position as a result of the action of a jack, thus making it possible to introduce the cylindrical part into a tube. When the pressure of the jack is relaxed, the deformable sectors return to the expanded position and thus ensure that the device is fastened in the tube.
However, when the aim is to obtain perfect positioning of a tool in relation to the end of a tubular piece fastened inside a tube passage in the tube plate, such a device for fastening inside a tube adjacent to the tube passage to be plugged may be inoperative, since the axes of the tubes or of the passage holes through the tube plate are not perfectly parallel.
In fact, the direction of the passage holes is not always absolutely perpendicular to the inlet face of the tube plate and during hydraulic tests on the steam generator and during the operation of this steam generator, the tube plate experiences deformations which result in slight relative inclinations of the successive bores passing through the plate.
Furthermore, the distance between the tube passages, which is perfectly defined and absolutely constant at the inlet face of the tube plate, undergoes variations within the tube plate which are attributable to the relative inclination of the axes of the passage holes.
The object of the invention is, therefore, to provide a device for welding a circular end part of a tubular piece fastened inside a bore opening onto a face of a plate, such as a team-generator tube plate having a set of tube passage holes arranged according to a regular network, this device being easily positionable by means of operations controlled remotely and in such a way that the welding operation is carried out under very good conditions.